1. Field of the Invention
The invention relates to a method for fabricating a semiconductor device, and more particularly, to a method for fabricating a PMOS transistor capable of preventing boron (B) ions of a gate from penetrating into a semiconductor substrate without deterioration of a gate oxide layer.
2. Brief Description of Related Technology
Polysilicon is commonly used as a material for forming a gate of a semiconductor device. Polysilicon has the physical properties, such as a high melting point, easiness of thin film forming and line pattern forming, required for the gate material. Conventionally, in both NMOS and PMOS transistors, the gate is formed of the polysilicon doped with an N-type dopant to simplify the process. Thus, a buried channel is formed in the PMOS transistor. However, as design rule of a semiconductor chip is decreased and low power consumption and high speed operation are required, the PMOS transistor having the buried channel has limitation. To overcome this limitation a dual gate process has been developed, in which the polysilicon doped with an N-type dopant is used in an NMOS region and the polysilicon doped with a P-type dopant is used in a PMOS region.
If the PMOS transistor is changed into a surface channel structure, a current on/off ratio and a role off property of a short channel threshold voltage are improved. To make the PMOS transistor as a surface channel structure, the gate is formed with the polysilicon doped with P+ dopant instead of the polysilicon doped with N+ dopant. Conventionally, to simplify the process, a polysilicon layer is doped with an N-type dopant, and a P-type dopant is then doped into the polysilicon layer in the PMOS transistor region by ion implantation or plasma doping, is used.
However, in this case, a boron (B) ion, a P-type dopant, passes through the gate oxide layer and penetrates into the surface of the semiconductor substrate. When the boron (B) ion penetrates into the semiconductor substrate, it changes the doping concentration inside the semiconductor substrate, which results in a phenomenon that can prevent the semiconductor device from operating normally. To prevent this phenomenon, the surface of the gate oxide layer is nitrified after the gate oxide layer is formed, which prevents the penetration of the boron (B) ion from the doped polysilicon layer. However, the boron (B) ion penetration is still not completely prevented. Also, the nitrification of the gate oxide layer can degrade the layer quality of the gate oxide layer. Consequently, the gate oxide layer can have a structure that is susceptible to a hot carrier effect, which results in deterioration of semiconductor device properties.